<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #parent {
            width: 100%;
            height: 500px;
            margin: 0 auto;
            display: flex;
            flex-direction: column;
        }
        .row {
            display: flex;
            flex: 1;
        }
        .item {
            flex: 1;
            border: 1px solid #000;
        }


    </style>
</head>
<body>
<div id="parent">
    <div class="row">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item">3</div>
    </div>
    <div class="row">
        <div class="item">4</div>
        <div class="item">5</div>
        <div class="item">6</div>
    </div>
    <div class="row">
        <div class="item">7</div>
        <div class="item">8</div>
        <div class="item">9</div>
    </div>
</div>

</body>
</html>